1. Field of the Invention
The present system is intended to provide a means for determining the direction of sources that emit radar or other rapidly modulated ultra-high frequency or microwave signals, and for doing so with moderately high accuracy (.+-.3.degree.) without recourse to large antennas or moving parts. Specifically, the system is proposed for use on naval vessels which sometimes must operate under strict emission controls (EMCON) and yet need to know the direction of various enemy ships, aircraft or missiles that are using radar to locate and attack them. Accurate direction finding against most types of enemy jammers is also possible.
2. Description of the Prior Art
Three catagories of direction finding (DF) systems have heretofore been utilized: (a) systems using narrow antenna beams that are scanned to determine the direction in which the incoming signal is the strongest; (b) generally equivalent systems using an antenna null that is scanned to reach a signal minimum; and (c) microwave interferometer systems.
If supplied with an antenna large enough to produce a narrow beam, the systems in category (a) can provide high accuracy and can function with very weak incoming signals; however, in their simple form such systems can function only in one direction at a given instant.
The systems of category (b), on the other hand, can supply reasonably high accuracy with a relatively small antenna, but require fairly strong incoming signals to function near the nulls, and again, are very limited in angle coverage at a given time. [The difference channels of monopulse receivers can be included in the (b) category, while the sum channels of the monopulse receivers relate to the (a) category systems.]
The category (c) systems overcome the common limitation of the other two systems and provide reception from sources anywhere within a wide angular coverage; however, if widely spaced antennas are used in the interferometer system to gain angular accuracy, ambiguities in the bearing data result and considerable complexity must generally be introduced in order to overcome the ambiguities. Also, interferometer systems require knowledge of the incoming signal frequency (or wavelength) to determine an angle off axis, and measurement of said frequency can present a problem, particularly when a plurality of incoming signals are intermixed. This problem also arises if the incoming signals are swept in frequency, as are the pulses of pulse compression ("chirped") radars.